Aircraft Accidents and FOD

I arrived at work one morning; my job is adjacent to a major international airport.  From my parking space, I looked out the windshield in time to see a clear, plastic, large-sized trash can liner tumbling in the wind twenty yards in front of me.  In my younger days, I might have chased it; even then I probably would not have succeeded in catching it because it was moving at a high rate of speed.  It leaped over the airport perimeter fence before I could even open my door, and it continued on, crossing Runway 13’s threshold.  Fortunately, there were no departing or arriving flights at that time of morning.

The colloquial term for the plastic bag is an acronym: FOD.  In aviation, we refer to such unrestrained pieces of garbage as Foreign Object Damage … FOD, for short.  The term appears innocuous, insipid; it’s almost as if I’m referring to FOD as a straight man to a more famous straight man: Elmer Fudd.  That would be a mistake, giving a sympathetic characteristic to such a destructive article.  FOD has cost the airlines millions of dollars annually in damage and delays.  Worse, FOD has cost the lives of many, a byproduct of carelessness.

In my younger days, my ramp crew and I embarked on FOD patrols, picking up junk that could be transformed into, e.g. a missile.  We would walk two arms’ lengths apart in grid patterns, eyes scanning the ground for just about anything, until the entire ramp was cleared.  FOD could be as large as a 2-X-4 piece of wood or as small as a #8 castellated nut; it could be a deer running across a runway in Alaska or a flock of geese taking out an A320’s engines at LaGuardia Airport.

One time, I worked on repairing a B727; a northbound gull had broke through the radome (nose) of the southbound B727, breached the thick metal pressure bulkhead and, after exhausting the energy breaking through the metal fuselage, still had enough kick to knock the Captain unconscious on final approach.  Think about that; hollow-boned birds with the density of a piece of chicken breast; birds that could be liquified in a Cuisinart, not only penetrated the bulkhead of a B727 at approach speed, but also shut down an A320’s engines in LaGuardia (US Air 1549).  It makes one wonder what damage a small unmanned aerial vehicle could do.

As mentioned, the plastic bag fluttering across my vision was FOD, but for a different reason; it can damage an airliner with engines running and can cause a catastrophic result if ingested during the critical stages of the take-off or landing phase of flight.

One morning when I worked for the FAA’s Flight Standards division, I was sitting with the flight crew conducting an enroute inspection.  We were in a Delta B737 and the ground crew had just finished pushing us out of the gate.  With engines started, the Captain waved off the tug and pushed the throttles forward to break ground resistance, our 200,000-pound airliner moved under its own power.  As he guided the airliner down the taxiway, I happened to look through the right window and saw a large plastic bag rolling tumbleweed-style just past the right wing.  I stated casually, “FOD off to starboard, two o’clock.”  The Captain slammed on the brakes, chopped the throttles and cued the first officer, who was already calling ramp control to report the bag.

No, the crew did not overreact; the Captain didn’t fear getting broadsided by a plastic bag.  Instead, this flight crew knew that for the two ‘vacuum cleaners’, aka engines, one on each wing, a plastic bag sucked into the engine inlet during taxi could cause, e.g. a compressor stall, damage the engine enough to be overhauled and cancel the flight.  What’s worse, if the bag made its way to the runway the consequences could be more severe: it could become the cause of an accident or serious incident, resulting in aircraft damage, personal injuries and, more importantly, loss of life … just from a large plastic bag.  The reason is that a plastic bag doesn’t get sliced to ribbons and vomited out the back of the engine; it could get spread across any number of the engine’s fan blades, blocking airflow.

During take-off, an engine’s fan section spins at a speed in the neighborhood of 3600 rotations per minute (RPM).  When FOD, like a plastic bag, gets sucked into the engine, it interrupts the flow of air that burns with fuel during combustion.  This airflow disruption – depending on the amount of bag-over-fan coverage – can disrupt combustion, causing a rapid overheat of the engine, thus damaging the engine’s internal parts.  The airstream’s force will prevent the bag from dislodging.  The engine would likely shut down on its own or be shut down by the pilot.  The internal engine damage incurred and/or decrease in airflow could be enough to prevent a restart.  The thrust from that engine would be lost just when it’s needed the most.

The FOD bag in the inlet could also starve the engine of air altogether; again, the engine shuts down, killing thrust on that side of the aircraft.  In the case of an inadvertent shutdown during these critical stages of take-off or landing, as the good engine continues at power, there is a longitudinal imbalance – the airliner turns toward the dead engine until the pilots can correct with rudder.  If they are landing in a crosswind, safety is compromised.  Since altitude is a pilot’s best friend, with less than 50 feet of altitude, the airliner would be in danger; all souls on board, in peril.

But FOD isn’t limited to the errant windblown plastic bag; damage isn’t restricted to just engines, yet the seriousness can be just as tragic.  So, what qualifies as FOD?  A bolt laying on the ground is FOD; pieces of scrap metal, glass, wood, rope, plastic strapping, hard rubber, even a chock used for blocking the support equipment’s tires.  A bolt ‘vacuumed’ into an engine is a missile of the worst kind when the rapidly-moving fan blades hit the object; powerful rotational energy is converted into comparable linear energy to move that bolt with the force of a bullet.  The high-energy fan blades themselves, after they initially impact the slow-moving bolt, become damaged; the bolt injures the fan blades with so much energy, the fan blades crack or sections break apart.  Out of balance, the rapidly-turning fan section vibrates in its shroud, damaging the internal engine components while still under high rotational speed.

How else does a discarded bolt or other piece of metal become a ‘dangerous object’?  Most drivers who have had their windshield chipped or broken by a pebble kicked up by an 18-wheeler can vouch for the power behind such a missile.  An 18-wheeler weighs around 80,000 pounds; as it presses the pebble down, it spits the pebble off in any direction.  The average narrow-body airliner can weigh 200,000 pounds or more, fully loaded; it can launch a piece of FOD at a wing, flight control or deeply imbed a bolt into its own tire tread.  The bolt can pierce the tire’s ply at critical moments, e.g. landing, when the landing weight is exaggerated.  A blown tire on landing can cause the aircraft to become unstable until it slows down.

Is the danger real?  On July 25, 2000, Air France flight 4590, an Aerospatiale Concorde crashed into a hotel during take-off; 113 people died – 109 on the aircraft and four on the ground.  An aircraft that departed shortly before lost a seventeen-inch long strip of stainless steel rub strip (FOD).  As the Concorde ran over the FOD, a main tire came apart; pieces of the tire struck the Concorde’s undercarriage at speeds upwards of 300 miles per hour.  The force of the tire pieces ruptured the number five fuel tank, which gushed fuel; the fuel ignited and the airliner could not increase power enough to gain any altitude.  It crashed, trailing a fuel-fed flame from the runway all the way to the accident site.

FOD is probably the least expected contributor to aviation damage because it’s innocent; by itself it can do no harm.  One can’t imagine a seventeen-inch piece of metal causing such a tragic end to over one hundred lives, but it did.  The discarded rub strip that led to the Air France accident was a hardy piece of metal; its purpose was to prevent damage by the vibrations of one engine cowl rubbing against another.  It was not as robust as a strengthening member of support structure, yet its effect was just as destructive, acting as the first domino in the series of events that doomed the airliner.

Consider also the Columbia Space Shuttle accident on February 1, 2003, during reentry.  Sixteen days earlier, on January 16, 2003, at 82 seconds into the shuttle’s launch (the vehicle has a velocity of about 4000 feet/second at this point), a piece of the external fuel tank’s foam struck the underside of the shuttle vehicle, breaching the shuttle’s left inboard wing thermal tiles.  On February 1st, with the crew unaware of the seriousness of the damage and based on poor management decisions, Columbia burned up on reentry, its fiery demise witnessed across several states.

The piece of insulation expelled off Columbia’s external fuel tank by itself could not cause much damage.  However, combined with the great velocity, the foam was just as dangerous to the thermal tiles as a brick through a plate glass window; its trajectory had the randomness of a bull in a china shop.  The resulting disaster proves it.

The chances of a used bolt becoming a missile, penetrating a fuel tank or tearing a composite structure, then becoming subject to the punishing effects of the airstream, increase through the improvements in technology.  The possibilities increase as later aircraft designs incorporate more lightweight composite materials and fuel tanks are built into the tail’s horizontal stabilizers.

Ramp inspections are boring; people get complacent, even treat such inspections as a joke.  Furthermore, many ramp crews are hired by contractors to meet that particular contractor’s – not the airline’s – manning needs.  They’re not taught the implications of FOD damage on an aircraft.  Gone are the days when everyone on the ramp, e.g. pilot, mechanic, ramp personnel, etc. was employed by the same airline; they all sported the same airline logo on their uniforms; they had the same pride in their airline and had a vested interest in each airliner’s safety. 

The bag flying across the parking lot?  It was not intentionally allowed to move freely through the active runway, just like the FOD that I saw on the Delta flight.  But that’s the problem with accidents: people didn’t intend for anything bad to happen; they just didn’t know.  If you’re reading this, now you know.  Hopefully I’ve awakened an awareness of dangerous circumstances that young aviators should be aware of; maybe I’ve made people aware of dangerous events they can prevent. 

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